The present invention relates to improved polyester fibers including flat yarns, tows, staple fibers and false twist yarns. More particularly, the invention relates to polyester fibers capable of being dyed with disperse dyes under normal pressure, having excellent color fastness and still having sufficient mechanical properties for practical use.
Generally polyester fiber, especially polyester fiber consisting essentially of polyethylene terephthalate, has many excellent properties such as tenacity, dimensional stability, thermal resistance and wash and wear property and many varied uses. On the other hand, polyethylene terephthalate fibers are poor in dyeability and it is therefore necessary to dye them under the conditions of a high temperature, e.g., about 130.degree. C., and a high pressure. Consequently, the production of such fibers suffers from the disadvantages that a special apparatus is required for dyeing. Moreover, use of such fibers in admixture with fibers such as wool, acrylic fibers and spandex fibers whose physical properties deteriorate upon dyeing under a high pressure and a high temperature, is limited.
Various improvements in dyeability of polyethylene terephthalate fibers under normal pressure have been proposed. A process in which accelerating agents called as carriers are employed in dyeing, for example, is known. However, the process has many disadvantages. More specifically, such carriers which are irritative and harmful to human body worsen working environmental sanitation at a dyeing factory and have difficulty in disposal of dyeing waste. Further uneven dyeing called as a carrier spot may be caused due to insufficiency of emulsification of the carriers and the carriers may remain in a dyed article to deteriorate the color fastness to light of the dyed article. Moreover, the carrier dyeing causes changes in the mechanical properties of the polyethylene terephthalate fiber such as a decrease in the tenacity and an increase in the elongation.
A copolymer of polyester with a compound having a metal sulfonate group or polyether has been considered a polyethylene terephthalate having an improved dyeability. Although such modified polyesters improve the dyeability, it is difficult to polymerize and spin them and the cost of the starting materials increases or the excellent mechanical and thermal properties possessed by polyethylene terephthalate and the color fastness may deteriorate. Consequently, the improvement in the dyeability resulting from such chemical modification detrimentally affects the inherent excellent thermal resistance and mechanical properties of polyethylene terephthalate, since the improvement is achieved by introducing a third component which can act as a dye receptacle for dyeing the polymer.